Which of the following combinations of goods are complements and which are substitutes? Can they be either

Question:

Which of the following combinations of goods are complements and which are substitutes? Can they be either in different circumstances? Discuss.
a) A mathematics class and an economics class
b) Tennis balls and a tennis racket
c) Steak and lobster
d) A plane trip and a train trip to the same destination
e) Bacon and eggs